Uranium and plutonium have gotten famous, or infamous, because they are used in atom bombs. We could have been saying that about another material—one that few actually know. Learn about the material that didn’t quite make it into The Bomb.

Neptunium is squashed between uranium and plutonium on the periodic table of elements. This gives us some idea of its possible use. Its neighbors both have isotopes that split under neutron bombardment, giving off a great deal of energy. They do this en masse when just a few kilograms are grouped together, which is why they make excellent weapons.

It would be reasonable to assume that neptunium has remained anonymous because of some chemical quirk that makes it unsuitable for bombs, but that’s not the case. Neptunium-237 could be made into a bomb. All anyone would need would be about sixty kilograms. Nuclear reactors produce tons of the stuff every year—albeit in tiny increments.

The main reason neptunium hasn’t been made into a bomb (other than the fact that now that we know it can be, access to it is regulated) is historical. Real work on a nuclear bomb began in the 1940s. This was just after Edwin M. McMillian and Philip H. Abelson succeeded in synthesizing neptunium-239. This isotope has a half-life of two and a half days, making it totally impractical as a weapon. For a while no one thought neptunium was a natural element. By the time people recognized that neptunium-237 could work as well as plutonium or uranium, there was no need for an element that was no better than the materials they already had the facilities for enriching. Today, neptunium is used in neutron detectors, but is otherwise considered totally useless.